This invention deals generally with the use of ultraviolet radiation to sterilize liquids and more specifically with a high power excimer lamp structure used to expose liquids to intense ultraviolet radiation to kill bacteria, even when the liquids are essentially opaque to ultraviolet radiation.
The exposure of liquids to ultraviolet radiation in order to sterilize them by killing bacteria is a long established technique. Many patents have been issued which are based on the ability of ultraviolet radiation to destroy bacteria, and such devices are common enough to be in use in many households and industries. Typically, such systems expose water to ultraviolet radiation by passing the water through an enclosure in which it is exposed to ultraviolet radiation.
One consideration which pervades all the prior art and is so well accepted that it is rarely even mentioned is that the treatment of water by exposure to ultraviolet radiation depends upon the water itself being significantly transparent to the ultraviolet radiation. The penetration of ultraviolet radiation through clear water typically may range from a few inches to more than a foot. Without such transparency to ultraviolet radiation, the purification of any liquid is very difficult because only the boundary of the liquid in actual contact with the source of radiation is affected by the radiation. Without the liquid's transparency to ultraviolet radiation, treatment requires very high intensity radiation and turbulence, so that the bacteria in the liquid can be killed in a very short time during which the turbulence assures that each portion of liquid is in direct contact with the ultraviolet radiation lamp.
However, achieving high intensity ultraviolet radiation is difficult. Traditional high intensity mercury lamps actually generate such a broad spectrum of radiation that the preferred wavelengths necessary to kill bacteria are only a small part of the power output. Moreover, conventional mercury lamps require an insulating sleeve over the lamp body which prevents direct contact between the lamp surfaces and the fluid being treated. On the other hand, excimer lamps, which have very narrow bandwidths at the appropriate wavelengths for sterilization of liquids, have been available only in relatively low power ratings. The historic limitations on the ultraviolet output power of existing excimer lamps have been rooted in their geometry.
Excimer lamps are essentially gas filled enclosures which are subjected to high voltage AC power by electrodes which are outside of, but in contact with, the enclosure. The lamp enclosures are constructed of a material such as quartz, so that they are transparent to ultraviolet radiation. When AC electrical power is applied, the lamp acts as the dielectric of a capacitor in which the electrodes are the plates of the capacitor, and, as in all capacitors, the dielectric provides all the impedance and uses all the power. For planar lamps, this means two metal electrodes are located in contact with the quartz gas filled planar envelope. For coaxial lamps, the envelope is usually formed of concentric cylinders sealed together at the ends, with the gas fill between the cylinders. The metal electrodes are then additional cylinders in contact with the outer surface of the outer quartz cylinder and inner surface of the inner cylinder.
Since existing lamps use one or more metal electrodes in contact with the lamp envelope, there is an inherent restraint on both lamp output and on lamp cooling. The electrode in contact with the outer surface of the lamp envelope has typically been a mesh which is partially transparent to ultraviolet radiation or a metal film which is so thin that some ultraviolet radiation passes through it. However, for high power operation such electrodes must also be capable of handling high electrical currents. That requires that they have significant volume in order to prevent limiting the electrical current or causing resistance heating. This high current requirement eliminates thin films and increases wire thickness in a mesh so greatly that the mesh blocks significant amounts of the ultraviolet radiation output. It then becomes a diminishing tradeoff in which the thick wire screens required for higher power levels block more of the ultraviolet radiation output which should be available for treatment of the liquid. Furthermore, the same mesh with large wires interferes with the cooling of the lamp surface, and when excimer lamps increase in temperature lamp efficiency and life are adversely affected.
The result has been an impasse which has prevented the use of excimer lamps in applications, such as the purification of opaque liquids, which require high ultraviolet radiation output.